We report femtosecond time-resolved pump-probe reliection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only A& modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with Aq vibrational modes, the quasiequilibrium nuclear A& coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of A& symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with A& symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(uot) dependence, where t = 0 is the time of arrival of the pump pulse peak, and uo is the vibrational frequency of the A& mode. These predictions agree well with our observations in Bi, Sb, Te, and Ti203. The fit of the experimental AR(t)/R(0) data to the theory is excellent.
We report the first systematic femtosecond pump-probe measurements of the electron-phonon coupling constant X in thin films of Cu, Au, Cr, Ti, W, Nb, V, Pb, NbN, and V36a. The agreement between our measured X, values and those obtained by other techniques is excellent, thus confirming recent theoretical predictions of Allen. By depositing thin Cu overlayers when necessary, we can extend this technique to nearly any metallic thin film.
Coherent phonons in Sb, Bi, Te, and Ti2O3 can be generated impulsively, and detected in the time domain through reflectivity modulation using 60 fs pulses of laser light at 2 eV. Experimental data for these opaque solids suggest that a direct Raman excitation mechanism is not responsible for coherent phonon generation. Rather, the excitation is attributed to an electronically induced displacement of the ion equilibrium coordinates.
We report time domain observations of coherent lattice vibrations in bismuth and antimony. Phonons are impulsively generated, and detected through reflectivity modulation with 70 fs pulses of laser light at 1.98 eV. With this technique, we demonstrate that coherent lattice oscillations can be studied by reflection in opaque materials, but with selection rules which may differ from conventional impulsive stimulated Raman scattering.
This letter introduces the general notion of significantly modulating the physical characteristics of a solid on a terahertz time scale with coherent lattice vibrations. We show, as an example, experiments in which coherent phonons are optically excited in single-crystal Ti2O3, which is a narrow-gap semiconductor at 300 K and gradually transforms into a semimetal at 600 K. Quantitative comparison of previous equilibrium measurements to our new transient measurements suggests that the induced coherent lattice vibration is significantly modulating the semiconductor-to-semimetal transition at 7 THz.
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